In the prior art, acoustic well diffusors are known, however, those of which applicants are aware are designed with an intent to shape and contour the reflected sound waves rather than to absorb them. In this regard, reference is made to applicants' two pending design patent applications, Ser. No. 774,882 filed Sept. 11, 1985 and Ser. No. 008,430 filed Jan. 29, 1987, on acoustical diffusors. Applicants are aware of no phase grating systems whose main purpose is that of sound absorbtion.